Known anti-skid brake control systems include means for monitoring the rotational speed of associated wheels and detecting when skidding is imminent and when it has been averted, and means for modulating the braking pressure supplied to the actuators of associated brakes for the avoidance of skidding.
The pressure modulating means is normally a pressure control mechanism having a first operational state in which it freely permits communication of pressure between the driver's control mechanism and the associated brake actuators to enable normal application and release of the brakes, and a second operational state in which it isolates the associated brake actuators from the driver's brake control mechanism and reduces braking pressure in the brake actuators for the avoidance of skidding of the associated wheels.
The wheel speed monitoring and skid detection functions are normally performed by electronic means which are arranged to change the operational state of the pressure modulating means from the first state to the second state when an incipient skid is detected and to return the pressure modulating means to its first operational state when skidding has been averted. The latter control function is essential for the maintenance of vehicle braking while the vehicle is in motion and the driver's demand for braking is sustained. Hence under braking conditions in which skidding of the associated wheels would otherwise occur the associated brakes are cyclically released (partially or wholly) and reapplied to avoid skidding and to maintain braking at or near an optimum level dictated by tyre and road surface conditions.
In systems as described, incipient skid corrective action is taken when loss of wheel speed exceeds some pre-determined criteria, such as, for example, detection of wheel deceleration exceeding a fixed threshold rate of deceleration. Detection of the subsequent state of successful skid avoidance is based upon the subsequent wheel speed rise characteristics satisfying criteria which ensure that wheel rotation is being sustained in a manner which is compatible with the changing speed of the braked vehicle.
In a vehicle, such as a multi-axle trailer, load transfer may occur between axles during braking because of the suspension mechanism of the trailer. As a result one axle is liable to lock before the other axles.